The disclosure of Japanese Patent Application No. HEI 11-136549 filed on May 18, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a power output unit, a method of controlling the power output unit, and a hybrid vehicle. More particularly, the present invention relates to a power output unit which has an internal combustion engine and motor-generators and in which an output shaft of the internal combustion engine, rotational shafts of the motor-generators and a drive shaft are mechanically connected to one another, a method of controlling the power output unit, and a hybrid vehicle.
2. Description of the Related Art
In recent years, various constructions have been proposed for a hybrid vehicle having motor-generators in addition to an internal combustion engine. A hybrid vehicle makes it possible to significantly reduce an amount of consumption of fossil fuel in comparison with a vehicle having a gasoline engine. As environmental problems become acute, social demands for hybrid vehicles grow. A parallel hybrid vehicle is one of such hybrid vehicles. In a parallel hybrid vehicle, both a power from an internal combustion engine and a power from an electric motor can be transmitted to a vehicle axle. FIG. 1 shows an example of the structure of a parallel hybrid vehicle. The hybrid vehicle shown in FIG. 1 has an engine 150 and motor-generators MG1, MG2. These three components are mechanically coupled to one another through a planetary gear 120. The planetary gear 120 is composed of three gears and has three rotational shafts respectively coupled to the gears. The gears constituting the planetary gear 120 are a sun gear 121 which rotates at the center, a planetary pinion gear 123 which rotates around the sun gear 121 while auto rotating, and a ring gear 122 which rotates around the planetary pinion gear 123. The planetary pinion gear 123 is pivoted on a planetary carrier 124. In the hybrid vehicle shown in FIG. 1, a crank shaft 156 serving as a drive shaft of the engine 150 is coupled to a rotational shaft of the planetary carrier 124, thus constituting a planetary carrier shaft 127. A drive shaft of the motor-generator MG1 is coupled to a rotational shaft of the sun gear 121, thus constituting a sun gear shaft 125. A drive shaft of the motor-generator MG2 is coupled to a rotational shaft of the ring gear 122, thus constituting a ring gear shaft 126. Furthermore, the ring gear 122 is coupled to a vehicle axle 112 through a chain belt 129 and a differential gear.
For the purpose of explaining the basic operation of a hybrid vehicle having such a construction, operation of the planetary gear 120 will first of all be described. In the planetary gear 120, if rotational speeds of two of the three rotational shafts and a torque of one of the three rotational shafts (hereinafter a rotational speed and a torque of a certain rotational shaft will comprehensively be referred to as a rotational state) are determined, rotational states of all the rotational shafts are determined. Although a relation among rotational states of the rotational shafts can be found out using a calculation formula which is well known to the community of mechanics, it can also be found out geometrically by means of an alignment chart.
FIG. 2 shows an alignment chart as an example. While the axis of ordinate shows rotational speeds of the rotational shafts, the axis of abscissa shows a relation in distance among gear ratios of the gears. A position C, which is an interior division point of 1: xcfx81 between the sun gear shaft 125 (S in FIG. 2) and the ring gear shaft 126 (R in FIG. 2), is defined as a position of the planetary carrier shaft 127. The value of xcfx81 represents a ratio (Zs/Zr) of the number of teeth of the sun gear 121 (Zs) to the number of teeth of the ring gear 122 (Zr). For the points S, C and R defined along the axis of abscissa, rotational speeds Ng, Ne and Nm of the rotational shafts are plotted respectively. According to the feature of the planetary gear 120, the three points that have thus been plotted never fail to be aligned along a single line. This line is referred to as an operation co-line. A line is uniquely determined if two points are specified. Thus, reference to this operation co-line makes it possible to calculate a rotational speed of one of the three rotational shafts from rotational speeds of the remaining two rotational shafts.
According to the feature of the planetary gear 120, when torque values of the rotational shafts are replaced with forces acting on the operation co-line, the operation co-line maintains its balance as a rigid body. As a concrete example, a torque acting on the planetary carrier shaft 127 is defined as Te. In this case, as shown in FIG. 2, a force corresponding to the torque Te is applied upwards to the operation co-line at the position C. A direction of application of the force is determined in accordance with a direction of the torque Te. Also, a torque Tp acting on the ring gear shaft 126 is applied downwards to the operation co-line at the position R. Tes and Tep shown in FIG. 2 are two equivalent forces obtained as a result of distribution of the torque Te according to the law of distribution of forces acting on a rigid body. The torque values Tes, Tep can be expressed by the following formulas (1) and (2).
Tes=xcfx81/(1+xcfx81)xc3x97Texe2x80x83xe2x80x83(1)
Tep=1/(1+xcfx81)xc3x97Texe2x80x83xe2x80x83(2)
In consideration of a condition that the operation co-line is balanced as a rigid body during application of those forces, it is possible to calculate a torque Tg to be applied to the sun gear shaft 125 by the motor-generator MG1 and a torque Tm to be applied to the ring gear shaft by the motor-generator MG2. The torque Tg becomes equal to the torque Tes, and the torque Tm becomes equal to a difference between the torque Tp and the torque Tep. The torque values Tg, Tm having such features are expressed by the following formulas (3) and (4) respectively.
Tg=xe2x88x92xcfx81/(1+xcfx81)xc3x97Texe2x80x83xe2x80x83(3)
Tm=Tpxe2x88x921/(1+xcfx81)xc3x97Texe2x80x83xe2x80x83(4)
While the engine 150 coupled to the planetary carrier shaft 127 rotates, the sun gear 121 and the ring gear 122 can rotate in various operation states with the aforementioned conditions on the operation co-line being satisfied. When the sun gear 121 rotates, it is possible to generate electricity in the motor-generator MG1 by means of a rotational power of the sun gear 121. When the ring gear 122 rotates, it is possible to transmit a power outputted from the engine 150 to the vehicle axle 112. In a hybrid vehicle having a construction shown in FIG. 1, a power outputted from the engine 150 is divided into a power that is mechanically transmitted to the vehicle axle 112 and a power that is converted into electric power through regeneration of one of the motor-generators MG1, MG2 (operating as a generator). Furthermore, the electric power that has been regenerated is used for power running of the other motor-generator (operating as an electric motor), whereby the vehicle can travel with a desired power outputted to the vehicle axle 112. Thus, when the hybrid vehicle constructed as shown in FIG. 1 travels, the motor-generators MG1, MG2 usually perform power running or regeneration. In this case, control is performed such that the electric power consumed during power running is balanced against the electric power generated during regeneration.
In the hybrid vehicle constructed as shown in FIG. 1, when controlling a travelling state of the vehicle, a torque requirement for the vehicle axle 112 (actually the ring gear shaft 126 mechanically coupled to the vehicle axle) is first of all determined from a vehicle speed and an accelerator opening degree. A power requirement to be outputted from the ring gear shaft 126 is determined from the torque requirement and the vehicle speed. Then, a power to be outputted from the engine 150 is determined such that the ring gear shaft 126 becomes capable of outputting the power requirement. A drive state of the engine 150 is controlled such that the engine 150 outputs the thus-determined power. Drive states of the motor-generators MG1, MG2 are controlled such that the aforementioned torque requirement is satisfied in the ring gear shaft 126 when the engine 150 outputs the aforementioned power. The motor-generators MG1, MG2 perform power running or regeneration, whereby a predetermined power outputted from the engine is converted into a desired rotational speed and a desired torque and outputted from the ring gear shaft 126, namely, from the vehicle axle 112.
In outputting a predetermined power corresponding to a power requirement that has been determined as a power to be outputted from the ring gear shaft 126, the engine 150 can assume various operation states (combinations of rotational speeds and output torque values). Thus, when the engine is controlled to output a predetermined power, an operation point of the highest efficiency is selected. Drive states of the motor-generators MG1, MG2 are controlled such that the engine is driven at the operation point.
After calculation of the operation point of the highest efficiency at the time when the engine 150 outputs the aforementioned predetermined power, a rotational speed and a torque at the operation point are set as a target rotational speed and a target torque of the engine 150 respectively. As described above, the drive shaft of the engine 150 is coupled to the rotational speed of the planetary carrier 124. Therefore, when the engine 150 outputs the predetermined power while being in operation at the operation point, the rotational speed of the planetary carrier shaft 127 is equal to the target rotational speed of the engine 150 that has been determined as described above. Also, the rotational shaft of the ring gear 122 is coupled to the drive shaft of the motor-generator MG2, and the ring gear 122 is mechanically coupled to the vehicle axle 112. Thus, a rotational speed of the ring gear shaft 126 can uniquely be calculated from the vehicle speed. Because the rotational shaft of the sun gear 121 and the drive shaft of the motor-generator MG1 are coupled to each other, the rotational speed of the motor-generator MG1 is equal to the rotational speed of the sun gear shaft 125. Once the rotational speed of the ring gear shaft 126 and the rotational speed of the planetary carrier shaft 127 are determined, a rotational speed of the sun gear shaft 125 can be calculated from the alignment chart shown in FIG. 2. Once rotational speeds of the rotational shafts coupled to the gears constituting the planetary gear 120 are thus determined, torque values to be outputted from the motor-generators MG1, MG2 are determined through a predetermined processing. If the engine 150 is controlled to output the predetermined power while controlling operation states of the motor-generators MG1, MG2 so that they are driven under such a condition, the engine 150 is operated in a state where the engine 150 demonstrates its highest efficiency. Thus, it is possible to realize a desired operation state in the hybrid vehicle.
The motor-generators MG1, MG2 perform power running or regeneration as described above, and can assume operation states corresponding to various rotational speeds and various output torque values. These rotational speeds and output torque values have threshold values. FIGS. 3 and 4 are explanatory views of output characteristics showing the threshold values of rotational speeds and output torque values of the motor-generators MG1, MG2. These threshold values are determined in accordance with the performance of the motors themselves or mechanical properties of the motors. Thus, in the hybrid vehicle, when an attempt is made to realize a desired operation state in the vehicle axle by converting a power outputted from the engine 150 through the planetary gear 120 and controlling the motor-generators MG1, MG2, operation states set for the motor-generators MG1, MG2 may exceed the threshold values of the motor-generators. That is, even if a power outputted from the engine 150 is within a range of the performance of the engine 150, an operation state determined for the motor-generator MG1 or MG2 may exceed the threshold values shown in FIGS. 3 and 4.
In the hybrid vehicle constructed as shown in FIG. 1, an operation state as shown in the alignment chart in FIG. 5 may arise when the vehicle speed is increased. In this state, the motor-generator MG2 regenerates electric power while the ring gear shaft 126 rotates in a positive direction. The motor-generator MG1 performs power running, whereby an electric power equivalent to the electric power regenerated by the motor-generator MG2 is consumed. An operation state of the motor-generator MG2 at the time when the hybrid vehicle assumes such an operation state is indicated as an example by a point xcex1 in FIG. 4, which is an explanatory view of output characteristics of the motor-generator MG2. If an accelerator pedal is depressed to increase a vehicle speed of the hybrid vehicle, control is performed to increase a rotational speed of the ring gear shaft 126, namely, a rotational speed of the motor-generator MG2. At this moment, the operation state of the motor-generator MG2, which is determined based on the aforementioned power requirement and an operation point where the engine 150 demonstrates its highest efficiency, becomes an operation state corresponding to a position marked with xcex2 in FIG. 4. That is, the threshold value of the operation state of the motor-generator MG2 is surpassed.
The operation demanded of the motor-generator MG2 becomes a state corresponding to the position marked with xcex2 in FIG. 4. In the case of excess of the threshold value, the motor-generator MG2 cannot assume such an operation state. Therefore, even in the case where the engine 150 still outputs sufficient power in comparison with the aforementioned power requirement, the vehicle speed cannot further be increased. Thus, in such a case, an upper limit of the vehicle speed is set not by a power that can be outputted by the engine 150 but by the performance of the motor-generator MG2.
In such a hybrid vehicle, if an attempt is made to realize a higher vehicle speed within the limit of an output state of the engine 150 in consideration of various travelling states, it is necessary to install motor-generators which are larger in size. That is, the motor-generators MG1, MG2 to be installed are sufficiently large in size, the motor-generators MG1, MG2 can be used for every possible travelling state within the range where the engine 150 can output power. However, if motor-generators larger in size are installed, the motor-generators occupy a greater space. Therefore, there is caused a problem of an increase in restrictions imposed on the design of the vehicle. Also, an increase in vehicle weight leads to a problem of deterioration of fuel consumption rate. Thus, it has been desired to realize a higher vehicle speed by sufficiently developing the performance of the engine without enlarging the motor-generators and thus improve the performance of the vehicle.
It is an object of a power output unit, a method of controlling the power output unit and a hybrid vehicle of the present invention to solve the aforementioned problems and sufficiently ensure the performance of the vehicle by sufficiently developing the performance of an engine without enlarging motors.
In a first aspect of the present invention, there is provided a power output unit comprising an engine having an output shaft, electric motors coupled to a drive shaft transmitting a power outputted from the engine through the output shaft to the outside, a power adjuster which is coupled to the output shaft and the drive shaft and which adjusts a power from the output shaft by means of electric power to transmit the power to the drive shaft, a power requirement calculator which calculates a power required for the engine, a rotational speed judging device which detects a rotational speed of the drive shaft and which compares the rotational speed with a permissible rotational speed of the electric motors, an operation state setting device which sets operation states of the electric motors such that the electric motors assume an output torque approximately equal to zero and a rotational speed equal to a rotational speed of the drive shaft and which sets an operation state of the engine based on the set operation states of the electric motors and the power requirement, and an operator which operates the engine, the power adjuster and the electric motors based on the operation state set by the operation state setting device.
In the thus-constructed power output unit of the present invention, the power adjuster, which is coupled to the drive shaft outputting a power to the output shaft of the engine and to the outside, transmits a power outputted from the engine to the drive shaft, and adjusts the drive power through exchange of electric power. This power output unit receives a rotational speed of the drive shaft and determines whether or not the rotational speed of the drive shaft has exceeded the threshold value of a rotational speed which is permissible when the electric motors coupled to the drive shaft output power. If it is determined that the threshold value of the rotational speed has been exceeded, operation states of the electric motors are set such that the electric motors assume an output torque approximately equal to zero and a rotational speed equal to the rotational speed of the drive shaft. Based on the set operation states of the electric motors and the power requirement for the engine, an operation state of the engine is set. The engine, the power adjuster and the electric motors are operated such that the electric motors and the engine assume the operation states that have been set.
This eliminates the possibility of the rotational speed of the drive shaft of the power output unit being limited by the performance of the electric motors. If sufficient power is outputted from the engine, it is possible to output a power composed of a desired rotational speed and a desired torque from the drive shaft while operating the electric motors such that output torque values of the electric motors become approximately equal to zero. This makes it possible to suppress the performance of the electric motors required to output a desired power from the drive shaft and reduce the size of the electric motors installed in the power output unit.
In the aforementioned aspect of the present invention, the power output unit may further comprise a secondary battery which can exchange electric power with the power adjuster and with the electric motors and a balance calculator which calculates an energy balance at least based on an energy loss generated during transmission of a power from the engine to the drive shaft and on a requirement for charge and discharge in the secondary battery. The operation state setting device corrects a power outputted from the engine by correcting a rotational speed of the engine based on the energy balance calculated by the balance calculator, when setting an operation state of the engine. The operator operates the engine and the power corrected by the operation state setting device.
In this construction, even in the case where the output torque of the drive shaft is affected by an output torque of the engine, the power outputted from the engine is corrected using a rotational speed of the engine. Thus, the torque outputted from the engine does not change. As a result, correction of the power outputted from the engine prevents the power outputted from the drive shaft from deviating from a desired value. In another aspect of the present invention, there is provided a power output unit comprising an engine having an output shaft, electric motors coupled to a drive shaft transmitting a power outputted from the engine through the output shaft to the outside, a power adjuster which is coupled to the output shaft and the drive shaft and which adjusts a power from the output shaft by means of electric power to transmit the power to the drive shaft, a power requirement calculator which calculates a power required for the engine, a rotational speed judging device which detects a rotational speed of the drive shaft and which compares the rotational speed with a permissible rotational speed of the electric motors, a torque setting device which sets an operation state of the engine based on the calculated power requirement and which sets output torque values of the electric motors based on the set operation state of the engine when the detected rotational speed is equal to or lower than the permissible rotational speed, a torque judging device which compares the set output torque values of the electric motors with a predetermined amount, an operation state setting device which sets operation states of the electric motors such that the electric motors assume an output torque approximately equal to zero and a rotational speed equal to a rotational speed of the drive shaft and which sets an operation state of the engine based on the set operation states of the electric motors and the power requirement, and an operator which operates the engine, the power adjuster and the electric motors based on the operation state set by the operation state setting device.
In the thus-constructed power output unit, the power adjuster, which is coupled to the drive shaft outputting a power to the output shaft of the engine and to the outside, transmits the power outputted from the engine to the drive shaft, and adjusts the transmitted power through exchange of electric power. This power output unit receives a rotational speed of the drive shaft and determines whether or not the rotational speed of the drive shaft has exceeded a threshold value of a rotational speed which is permissible when the electric motors coupled to the drive shaft output power. If it is determined that the threshold value of the rotational speed has not been exceeded, an operation state of the engine is set based on the power requirement for the engine. Output torque values of the electric motors are set based on the operation state of the engine that has thus been set. If it is determined that the thus-set output torque values of the electric motors have exceeded the threshold value, operation states of the electric motors are set such that the electric motors assume an output torque lower than the threshold value and a rotational speed equal to the rotational speed of the drive shaft. Also, an operation state of the engine is set based on the set operation states of the electric motors and the power requirement. Further, the engine, the power adjuster and the electric motors are operated such that the electric motors and the engine assume the set operation states.
This eliminates the possibility of the power outputted from the drive shaft being limited by the performance of the electric motors. If sufficient power is outputted from the engine, it is possible to output a desired torque and a desired rotational speed from the drive shaft while operating the electric motors such that output torque values of the electric motors are confined to the range of the threshold value. This makes it possible to suppress the performance of the electric motors required to output a desired power from the drive shaft and further reduce the size of the electric motors installed in the power output unit.